Ceramic dielectrics compositions

ABSTRACT

A sinterable ceramic composition which comprises a barium titanate based dielectric precursor powder which has a temperature coefficient of capacitance, TC, of ±15% over the temperature range -55° to 125° C., in admixture with from 0.25 to 2.0% by weight of an additive which is selected from the group consisting of ZrSiO 4 , Al 2  O 3 , SiO 2 , precursors therefor and mixtures thereof, the composition having an average particle size in the range of from 0.6 to 0.8 micrometers and, when fired, having a dielectric constant of above 2500, a TC of ±15% over the temperature range of -55° to +125° C. and a porosity of less than 0.7% with no pores greater than 3.5 micrometers average diameter.

BACKGROUND OF THE INVENTION

The present invention relates to ceramic dielectric compositions and, inparticular, to ceramic dielectric compositions which, when sintered,have dielectric constants between 2400 and 3100, low dissipation factor(DF), e.g., below about 2%, high insulation resistance capacitance (RC)products, e.g. above 5000 ohm--farad at 25° C. and above 1000 ohm-faradat 125° C.; stable temperature coefficients (TC), in which thedielectric constant does not alter from in base value at 25° C. by morethan 25% over a temperature range from -55° to 125° C.; a near zeroporosity level; and an average particle size of less than 0.8 microns.

The ceramic compositions of the present invention are useful inmanufacturing multilayer ceramic capacitors (hereinafter MLC) whichrequire a high capacitance and which typically have a relatively smallsize. The ultra low porosity of these compositions, combined with thesubmicron powder particle size, enables thinner layers to be made in theMLC, which results in a higher capacitance and reliability of thedevice. MLCs are commonly made by casting or otherwise forminginsulating layers of a dielectric ceramic powder upon which conductingmetal electrode layers, usually consisting of a palladium/silver alloy,are deposited. The ceramic composition must then be fired attemperatures greater than or equal to 1280° C. to form the MLC device.

It is well known that temperature stable ceramic dielectrics may beprepared by mixing pure barium titanate with other minor additives forcontrol of the final dielectric properties. Using commercially availablehigh purity barium titanate produced by chemical coprecipitation by thesolid state method, downward Curie shifters can be added to shift theCurie peak of the BaTiO₃ from about 125° C. to room temperature where itis desirable to have a high dielectric constant. The stability of thedielectric constant over a wide range of temperatures, its insulationresistance, reliability, and other factors influence the finalingredients to be used in a dielectric composition.

Typically barium titanate powders are normally physically characterizedas having an average particle sizes of greater than 1.0 micrometers indiameter. This can be achieved by either air impact milling or wetmilling. As MLC technology is going towards greater miniaturization, itis more desirable to have thinner dielectric layers which will achieve ahigher capacitance with the same design. Submicron particle size powdersare desirable for thinner and thinner layers. Typical ceramic dielectriccompositions which are disclosed in the prior art, such as U.S. Pat.Nos. 4,882,305 and 4,816,430, lose their stability, especially TCcharacteristics, as the particle size of the dielectric powder isreduced to less than 0.7 micrometers. Also, there is a small populationof large size pores apparent in MLCs made with dielectric powders of thestandard particle size. Although these pores do not affect theelectrical properties and reliability of MLCs having dielectric layersof thickness 25 micrometers, the large size pores become increasinglyundesirable as the dielectric layers become thinner and thinner. Theselarge pores of approximately 5 to 7 micrometers in size will eventuallycause problems of electrical degradation and reliability.

It is known in the art that when the particle size of barium titanatebased compositions is reduced by milling, an improvement of porosity isachieved. However, currently available milling methods have side affectssuch as a noticeable clockwise rotation of the TC curve. Attempts tocounter this effect by varying the ingredients of a standard high fireX7R composition (such as the ratio or level of the Nb₂ 0₅ /CoOadditives) have been unsuccessful to date. A high fire X7R prior artcomposition was studied but could not maintain a stable X7R TC, which isthat the capacitance does not vary by more than ±15% over thetemperature range of -55° C. to 125° C. when the particle size wasreduced to below 0.8 micrometers. A typical example is given below.

                  TABLE I                                                         ______________________________________                                        Average particle                                                                          1.3     1.02    0.8    0.72  0.58                                 size (μm)                                                                  Dielectric Constant                                                                       3458    3595    3767   4047  4386                                 TC (%) @                                                                      -55° C.                                                                            -2.60   -0.8    -6.4   -18.5 -34.9                                 25° C.                                                                            0       0       0      0     0                                     85° C.                                                                            -4.4    -6.5    -10.4  -16.6 -23.4                                105° C.                                                                            -3.2    -6.1    -11.3  -19   -29.4                                125° C.                                                                            7.7     2.3     -6.1   -17.1 -32.1                                ______________________________________                                    

Without changing the dielectric composition, the dielectric propertieslisted above indicate that when the average particle size is reduced toless than 0.8 micrometers the TC can no longer meet the ElectronicIndustry Association X7R TC specification. It is noted that the datagiven above in Table 1 are for disc ceramic capacitors. It is well knownto those who are familiar with the art that the TC of a ceramicmultilayer capacitor is expected to be at least 4% more negative on thehigh temperature side due to the reaction of the ceramic with theinternal electrodes and due to the dielectric thickness. Therefore,based on Table 1 data, powders with a particle size of less than 0.8micrometers will have a TC lower than -15% when applied in MLCs eventhough the TC is within -15% in the disc.

SUMMARY OF THE INVENTION

We have now developed a submicron particle size high fire X7R ceramicdielectric composition which exhibits dielectric constant stability overa wide temperature range, which will exhibit a stable temperaturecoefficient of capacitance over the temperature range of -55° to 125° C.when milled to an average submicron particle size of less than 0.8micrometers and which has reduced porosity in the fired ceramic for thepurpose of providing thinner dielectric layers and higher reliability.

Accordingly, in one aspect of the present invention there is provided asinterable ceramic composition which comprises a barium titanate baseddielectric precursor powder which has a temperature coefficient ofcapacitance, TC, of ±15% over the temperature range -55° to 125° C., inadmixture with from 0.25 to 2.0% by weight of an additive which isselected from the group consisting of ZrSiO₄, Al₂ O₃, SiO₂, precursorstherefor and mixtures thereof, the composition having an averageparticle size in the range of from 0.6 to 0.8 micrometers and, whenfired, having a dielectric constant of above 2500, a TC of ±15% over thetemperature range of -55° to +125° C. and a porosity of less than 0.7%with no pores greater than 3.5 micrometers average diameter.

In another aspect the present invention provides a method for thepreparation of a sintered composition having an average particle size inthe range of from 0.6 to 0.8 micrometers and, when fired, having adielectric constant of above 2500, a TC of ±15% over the temperaturerange -55° to 125° C. and a porosity of less than 0.7% with no poresgreater than 3.5 micrometers, which method comprises milling a bariumtitanate precursor powder having a temperature coefficient ofcapacitance, TC, of ±15% over the temperature range -55° to 125° C. andhaving an average particle size of above 0.8 micrometers with from 0.25to 2% by weight of an additive which is selected from the groupconsisting of ZrSiO₄, Al₂ O₃, SiO₂, precursors therefor and mixturesthereof to an average particle size of from 0.6 to 0.8 micrometers.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an SEM of a polished section of an MLC made with a standardK4000 X7R barium titanate dielectric material;

FIG. 2 is an SEM of a polished section of a MLC oxide with thecomposition of Examples 42; and

FIG. 3 is a comparison of the pore size distributions for the K4000composition and the composition of Example 42.

DETAILED DESCRIPTION OF THE INVENTION

The compositions of the present invention, when fired, have a porosityof less than 0.7% with no pores greater than 3.5 micrometers averagediameter. In order to determine the degree of porosity, number of poresand pore size distributions, polished sections of the sinteredcompositions, for example as MLCs, are prepared and theirmicrostructures analysed by SEM coupled with an image analyser.

The compositions of the present invention preferably include theadditive ZrSiO₄, Al₂ O₃, SiO₂, or precursors therefor, or mixturesthereof, in an amount of from 0.5 to 1.0% by weight.

The compositions of the present invention, when fired, preferably have adielectric constant in the range of from 2400 to 3100 at 25° C., adissipation factor of less than 2% and an insulation resistance andcapacitance produce of about 5000 ohm-farads at 25° C. and about 1000ohm-farads at 125° C.

The composition of the present invention has an average particle size inthe range of from 0.6 to 0.8 micrometers, preferably about 0.7micrometers.

The barium titanate precursor composition which is used in the presentinvention has a temperature coefficient of capacitance, TC, of ±15% overthe temperature range -55° to +125° C. The precursor compositionpreferably is of high purity and to attain the desired TC value includestherein additives of niobium pentoxide (Nb₂ O₅), cobalt oxide (CoO), andmanganese oxide (MnO). As previously described herein these precursorcompositions may be prepared according to the teaching of U.S. Pat. Nos.4,882,305 and 4,816,430. The preferred stoichiometric ratio for thebarium titanate is about 0.995.

In carrying out the method of the present invention the barium titanateprecursor powder and the additive are milled together using conventionalmilling procedures, preferably for a period of time of from 2 to 8hours.

The ceramic compositions of the present invention may be formed intoslurries with appropriate binder compositions for the preparation oftapes from which MLCs are prepared by conventional techniques. It shouldbe noted that any conventional ceramic binder composition may be used inthis invention which is compatible with the powders used for thisinvention and which provides a vehicle for dispersing the ceramicparticles and holding them together when the solvent is removed.Suitable binder compositions are described in "Ceramic Processing BeforeFiring", Ch. 19. G. Y. Onoda, Ir., et al John Wiley & Sons (1978).Polyvinyl alcohol in water and polyvinyl butyl in methyl ethyl ketonealcohol are examples of other suitable equitable binder compositions.

The present invention will be more particularly described with referenceto the following Examples.

EXAMPLES 1 to 12

Approximately 650 grams of a commercially available K4200 X7R powder,such as TAMTRON X7R412H, were added with 6.5 grams or 13.0 grams of anadditive selected from ZrSiO₄ (zirconium silicate), Al₂ O₃, SiO₂ andCaZrO₃ to 800 cc of distilled water, and 5000 grams of 0.5 cm diameteryttria stabilized ZrO₂ media in a plastic ball mill jar. Thecompositions were milled for 2 or 8 hours. The average particle size ofthe above powder compositions was about 1.0 micrometers after 2 hoursmilling and about 0.7 micrometers after 8 hours milling. The wetslurries were then dried into a cake and ground with a mortar andpestle. 2.4 to 4.0 cc of a binder solution including 30 wt % corn syrupsolids and 70 wt % distilled water was mixed with 30 grams of eachceramic powder in a mortar and pestle which was granulated through a 40mesh nylon screen. Discs of the resultant mixture having a diameter of1.27 cm and a thickness of 0.1 to 0.15 cm were pressed at a pressure ofabout 38,000 pounds per square inch in a stainless steel disc. The discswere placed on a stabilized zirconia setter and fired at 1300° to 1340°C. for 2 hours. After cooling, silver electrodes were painted on thediscs which were then fired at 815° C. to sinter the electrodes. Thecapacitance (C), the dissipation factor (DF), and the capacitance changewith temperature as compared to the reference at 25° C. were thenmeasured with a model HP4274A capacitance bridge at 1 KHz measurementfrequency, from -55° to 125° C. at about 20° C. intervals. Thedielectric constant K, of each sample was then calculated from thefundamental capacitance equation:

    K=5.66 * Ct/D.sup.2

where

K=dielectric constant of the sample

t=thickness of the disc in inches

D=diameter of the disc in inches

C=capacitance of the disc in picofarads.

It is noted that TAMTRON X7R412H is a commercially available dielectricpowder. It is manufactured by blending high purity barium titanate,technical grade fine particle size niobium pentoxide, and technicalgrade fine particle size cobalt oxide. Details have been disclosed inU.S. Pat. No. 4,882,305.

Table II lists the dielectric properties of the above disc samples. Itdemonstrates that the addition of ZrSi0₄, Al₂ O₃ and SiO₂ in smallamounts stabilizes the TC even when the particle size of the powder isreduced to about 0.7 micrometers (Compare Examples 1, 2, 3, 4, 5, 6, 7and 8). The addition of CaZrO₃ does not show any advantage (Examples 9,10, 11 and 12).

                                      TABLE II                                    __________________________________________________________________________    % dopant     milling time                                                                         K. 1 KHz                                                                            % DF                                                                              TC @ -55° C.                                                                    TC @ 85° C.                                                                   TC @ 105° C.                                                                    TC @ 125°       __________________________________________________________________________                                                           C.                     Example 1                                                                           0      2      4015  1.01                                                                              -8.3     -7.4   -8.0     0.0                    Example 2                                                                           0      8      4291  1.16                                                                              -22.7    -13.5  -15.6    -11.9                  Example 3                                                                           1% ZrSiO.sub.4                                                                       2      3038  0.91                                                                              -7.1     -0.5   0.3      6.6                    Example 4                                                                           1% ZrSiO.sub.4                                                                       8      3084  0.98                                                                              -8.2     -5.2   -5.9     -2.2                   Example 5                                                                           1% Al.sub.2 O.sub.3                                                                  2      3107  0.88                                                                              -6.3     -1.8   -1.1     6.4                    Example 6                                                                           1% Al.sub.2 O.sub.3                                                                  8      3163  0.98                                                                              -9.0     -4.9   -5.4     -0.8                   Example 7                                                                           1% SiO.sub.2                                                                         2      3019  0.88                                                                              -7.9     -2.3   -2.2     3.2                    Example 8                                                                           1% SiO.sub.2                                                                         8      3113  0.98                                                                              -10.5    -5.8   -6.9     -3.8                   Example 9                                                                           1% CaZrO.sub.3                                                                       2      3695  0.96                                                                              -4.4     -5.7   -5.7     2.3                    Example 10                                                                          1% CaZrO.sub.3                                                                       8      4477  1.38                                                                              -28.3    -19.8  -25.4    -25.9                  Example 11                                                                          2% CaZrO.sub.3                                                                       2      3576  0.97                                                                              -4.0     -6.6   -7.3     -1.6                   Example 12                                                                          2% CaZrO.sub.3                                                                       8      4234  1.22                                                                              -21.9    -15.5  -18.8    -16.6                  __________________________________________________________________________

EXAMPLES 13 to 22

Using the same mixing and milling procedures as detailed above forExamples 1 to 12 but using another commercially available high fire X7Rcomposition, TAMTRON X7R402H, the compositions of Examples 13 to Example22 were prepared so that the amount of ZrSiO₄ was from 0 to 5 weightpercent. The compositions were milled for 10 hours so that the averagepowder particle size was about 0.7 micrometers. It is to be noted thatTAMTRON X7R402H was manufactured by blending high purity bariumtitanate, technical grade fine particle size niobium pentoxide,technical grade fine particle size cobalt oxide, and high puritytechnical grade manganese carbonate. Details have been disclosed in U.S.Pat. No. 4,816,430.

Table III lists the dielectric properties of discs prepared from thevarious compositions according to the procedure of Examples 1 to 12. TheTable indicates that when the ZrSiO₄ addition level is less than 0.25weight %, such as Example 13, the TC is not stable. When the ZrSiO₄addition level is above 2.0 weight %, such as Examples 20, 21 and 22,either the dielectric constant is low or the TC is not stable, or both.They are, therefore, outside the scope of the present invention.

                                      TABLE III                                   __________________________________________________________________________    % ZrSiO.sub.4                                                                             K at 1 KHz                                                                          % DF                                                                              TC @ -55° C.                                                                   TC @ 85° C.                                                                   TC @ 105° C.                                                                   TC @ 125° C.              __________________________________________________________________________    Example 13                                                                          0     3758  0.9 -9.7    -9.8   -11.0   -6.5                             Example 14                                                                          0.25  3337  0.83                                                                              -5.4    -6.8   -7.8    -3.3                             Example 15                                                                          0.5   3156  0.82                                                                              -4.8    -5.2   -5.7    -1.6                             Example 16                                                                          0.75  2972  0.84                                                                              -4.7    -3.9   -4.3    -0.5                             Example 17                                                                          1.0   2814  0.85                                                                              -4.9    -3.0   -3.3    0.3                              Example 18                                                                          1.5   2557  0.86                                                                              -5.5    -2.0   -2.3    0.6                              Example 19                                                                          2.0   2341  0.84                                                                              -5.7    -1.8   -2.3    -0.1                             Example 20                                                                          3.0   2023  0.85                                                                              -3.6    -3.7   -4.9    -4.0                             Example 21                                                                          4.0   1824  0.86                                                                              2.7     -9.5   -12.1   -12.6                            Example 22                                                                          5.0   1746  0.92                                                                              12.6    -19.8  -24.8   -27.6                            __________________________________________________________________________

EXAMPLES 23 to 32

Using the same procedure as described in Examples 1 to 12 using acommercially available powder TAMTRON X7R412H, the compositions ofExamples 23 to 32 were prepared with additions of 0 to 5.0 weightpercent of ZrSiO₄ and tested in the same way as for Examples 1 and 2.Table IV lists the results. It indicates that when the ZrSiO₄ additionis less than 0.25 weight percent, such as Example 23, the TC is notstable. When the ZrSiO₄ addition level is above 2.0 weight percent, suchas Examples 30, 31 and 32, either the dielectric constant is low or theTC is not stable, or both.

                                      TABLE IV                                    __________________________________________________________________________    % ZrSiO.sub.4                                                                             K at 1 KHz                                                                          % DF                                                                              TC @ -55° C.                                                                   TC @ 85° C.                                                                   TC @ 105° C.                                                                   TC @ 125° C.              __________________________________________________________________________    Example 23                                                                          0     4237  1.12                                                                              -22.8   -13.9  -16.0   -12.3                            Example 24                                                                          0.25  3693  1.00                                                                              -14.7   -10.3  -11.7   -7.7                             Example 25                                                                          0.5   3452  0.98                                                                              -10.4   -7.8   -8.8    -4.6                             Example 26                                                                          0.75  3240  0.96                                                                              -8.7    -6.3   -7.1    -3.1                             Example 27                                                                          1.0   3058  0.95                                                                              -8.2    -5.3   -6.0    -2.4                             Example 28                                                                          1.5   2783  0.95                                                                              -8.2    -4.3   -5.1    -2.2                             Example 29                                                                          2.0   2555  0.97                                                                              -8.1    -4.2   -5.1    -2.9                             Example 30                                                                          3.0   2255  0.95                                                                              -5.2    -7.0   -8.9    -8.4                             Example 31                                                                          4.0   2114  1.03                                                                              0.9     -14.0  -18.1   -19.7                            Example 32                                                                          5.0   2252  1.53                                                                              -6.0    -25.0  -32.8   -37.7                            __________________________________________________________________________

EXAMPLES 33 to 41

Using the same procedure as described in Examples 23 to 32, 8.0 grams ofZrSi0₄ was added to 800 grams of TAMTRON X7R412H powder and the mixturemilled in the same way as described in Examples 1 to 12 except thatafter about every 2 hours of milling a small portion of the slurry wastaken and dried into a powder. Disc capacitors were when prepared fromthe thus dried powder and tested in the same way as for Examples 1 to12, Table V lists the results. It demonstrates that with the addition ofZrSiO₄, a X7R powder having a 0.7 micrometers average particle size anda stable TC can be obtained.

                                      TABLE V                                     __________________________________________________________________________    Milling     Avg. Part                                                                           K at      TC@  TC@ TC@ TC@                                  Time (Hr.)  Size (μm)                                                                        1 KHz % DF                                                                              -55° C.                                                                     85° C.                                                                     105° C.                                                                    125° C.                       __________________________________________________________________________    Example 33                                                                          1     1.08  2999  0.93                                                                              -8.4 0.0 1.3 8.3                                  Example 34                                                                          2     1.01  2987  0.97                                                                              -8.3 -0.4                                                                              0.7 7.4                                  Example 35                                                                          4     0.88  2996  0.96                                                                              -7.96                                                                              -1.09                                                                             -0.1                                                                              6.1                                  Example 36                                                                          6     0.80  3006  0.97                                                                              -7.5 -1.7                                                                              -1.1                                                                              4.4                                  Example 37                                                                          8     0.76  3033  0.97                                                                              -7.9 -2.7                                                                              -2.5                                                                              2.4                                  Example 38                                                                          10    0.74  3032  1.03                                                                              -8.6 -3.9                                                                              -4.2                                                                              -0.1                                 Example 39                                                                          12    0.69  3060  1.03                                                                              -9.0 -5.0                                                                              -5.6                                                                              -2.2                                 Example 40                                                                          16    0.62  3189  1.12                                                                              -12.3                                                                              -9.0                                                                              -10.9                                                                             -9.4                                 Example 41                                                                          20    0.58  3340  1.22                                                                              -18.7                                                                              -13.6                                                                             -17.1                                                                             -17.6                                __________________________________________________________________________

EXAMPLES 42 to 47

650 grams of a uniformly blended ceramic composition TAMTRON X7R402H wasprepared with an addition of 0.3 to 0.8 weight percent of ZrSiO₄ in thesame way as described in Examples 1 to 12. The average particle size ofthe powders was about 0.7 micrometers. About 400 grams of the thusprepared 0.7 micrometer powder composition was added together with 218grams of a binder solution made by uniformly mixing and dissolving 186grams dioctylphthalate, 90 grams Nuostabe V-1444 (Nuostabe V-1444 is analkali ion free organic solvent dispersing agent available from NuodexCo. Inc. New Jersey), 2597 ml ethanol, 270 ml toluene, and 372 gramsButvar B-76 vinyl resin (Butvar B-76 is a binder comprising a mixture ofpolyvinyl butyl, polyvinyl alcohol and polyvinyl acetate available fromMonsanto Corp.).

The slurry was milled for 16 hours, discharged and filtered through a 44micrometer screen. The slurry had a viscosity of about 1500 to 3000centipoise and was then deaired and tape cast, in accordance withstandard techniques known in the industry. The tape was cast to athickness of about 30 to 37.5 micrometers. The tape was converted toMLCs having internal electrodes comprised of about 70 wt % palladium and30 wt % silver. The capacitors were then preheated, or baked out, at260° C. for 48 hours on a stabilized zirconia setter and then sinteredat 1290° to 1340° C. for about one to four hours.

Six composition are listed in Table VI that show ZrSiO₄ levels varyingfrom 0.3 to 0.8 weight percent. The MLC results of these sixcompositions are also listed in Table VI. It demonstrates that with 0.3to 0.8 percent addition of ZrSiO₄ to a conventional K4000 X7R typedielectric powder a powder composition with a small particle size, suchas 0.7 micrometer having a high dielectric constant, greater than 2500,and stable TC can be obtained.

                                      TABLE VI                                    __________________________________________________________________________          Avg. Part.                                                                          %   MLC      TC@  TC@ TC@ TC@                                           Size (μm)                                                                        ZrSiO.sub.4                                                                       K value                                                                            % DF                                                                              -55° C.                                                                     85° C.                                                                     105° C.                                                                    125° C.                          __________________________________________________________________________    Example 42                                                                          0.7   0.3 3302 1.77                                                                              5.3  -10.4                                                                             -12.3                                                                             -9.7                                    Example 43                                                                          0.7   0.4 2896 1.69                                                                              5.1  -9.4                                                                              -11.1                                                                             -8.4                                    Example 44                                                                          0.7   0.5 2954 1.62                                                                              4.2  -8.7                                                                              -10.3                                                                             -7.6                                    Example 45                                                                          0.7   0.6 3461 1.91                                                                              -2.9 -10.2                                                                             -12.9                                                                             -11.6                                   Example 46                                                                          0.7   0.75                                                                              2892 1.63                                                                              3.7  -7.1                                                                              -8.4                                                                              -5.6                                    Example 47                                                                          0.7   0.8 2913 1.85                                                                              -0.8 -9.8                                                                              -11.6                                                                             -9.2                                    __________________________________________________________________________          Avg. Part                                                                           %   MLC  K plate                                                                            RC @ RC @ BDV                                             Size (μm)                                                                        ZrSiO.sub.4                                                                       K    K    25° C.                                                                      125° C.                                                                     (v/25 μm)                              __________________________________________________________________________    Example 42                                                                          0.7   0.3 3302 3256 22200                                                                              3060 873                                       Example 43                                                                          0.7   0.4 2896 3126 14600                                                                              3620 746                                       Example 44                                                                          0.7   0.5 2954 3075 18500                                                                              3666 916                                       Example 45                                                                          0.7   0.6 3461 3388 8070 3190 850                                       Example 46                                                                          0.7   0.75                                                                              2892 2860 17300                                                                              3750 914                                       Example 47                                                                          0.7   0.8 2913 3110 7700 3150 822                                       __________________________________________________________________________

Furthermore, MLCs made from powders such as Examples 42 to 47 have asuperior microstructure with nearly zero internal porosity. Examples areillustrated in FIG. 1 which is a fracture surface view of an MLC madefrom a conventional K4000 X7R type powder, and in FIG. 2, which is afracture surface view of an MLC made the powder of Example 42. Theconventional K4000 X7R powder had a mean porosity determined by themethod as described herein of 1.56% (standard deviation=0.18) and thelow porosity powder made according to Example 42 had a mean porosity of0.51% (standard deviation-0.15). The low valves of standard deviation ineach case indicates that these results are representative of theporosity in the samples. The pore size distributions for a similar crosssectional area of each sample are shown in FIG. 3. The low porositycomposition of Example 42 has fewer pores and there are no pores >3.5micrometers in diameter.

It is to be understood that the present invention is not limited to thespecific embodiments described in the Examples. More specifically,although the Examples are illustrated using commercially availabledielectric compositions such as TAMTRON X7R412H and TAMTRON X7R402H, itis to be understood that other compositions which have an X7Rspecification may also be used in the present invention. Furthermore,although most of the Examples illustrates the addition of ZrSiO₄ to thecompositions it is to be understood that additions of ZrO₂ /SiO₂, Al₂ O₃or SiO₂ may also be used.

We claim:
 1. A sinterable ceramic composition which comprises a bariumtitanate based dielectric precursor powder which has a temperaturecoefficient of capacitance, TC, of ±15% over the temperature range -55°to 125° C., in admixture with from 0.25 to 2.0% by weight of an additivewhich is selected from the group consisting of ZrSiO₄, Al₂ O₃, SiO₂,precursors therefor and mixtures thereof, the composition having anaverage particle size in a range of from 0.6 to less than 0.8micrometers and, when fired, having a dielectric constant of above 2400,a TC of ±15% over a temperature range of -55° to +125° C. and a porosityof less than 0.7% with no pores greater than 3.5 micrometers averagediameter.
 2. A composition according to claim 1 which comprises from 0.3to 1.0% of the additive.
 3. A composition according to claim 1 which hasan average particle size of about 0.7 micrometers.
 4. A compositionaccording to claim 1 which when fired has a dielectric constant in arange of from 2400 to 3100 at 25° C.
 5. A composition according to claim4 which when fired has a dissipitation factor of less than 2% and aninsulation resistance and capacitance product of about 5000 ohm-faradsat 25° C. and about 1000 ohm-farads at 125° C.
 6. A compositionaccording to claim 1 wherein the barium titanate precursor powderincludes therein niobium pentoxide, cobalt oxide and manganese oxide. 7.A method for the preparation of a sintered composition having an averageparticle size in the range of from 0.6 to 0.8 micrometers and, whenfired, having a dielectric constant of above 2500, a TC of ±15% over thetemperature range -55° to 125° C. and a porosity of less than 0.7% withno pores greater than 3.5 micrometers, which method comprises milling abarium titanate precursor powder having a temperature coefficient ofcapacitance, TC, of ±15% over the temperature range -55° to 125° C. andhaving an average particle size of above 0.8 micrometers with from 0.25to 2% by weight of an additive which is selected from the groupconsisting of ZrSiO₄, Al₂ O₃, SiO₂, precursors therefor and mixturesthereof to an average particle size of from 0.6 to 0.8 micrometers.
 8. Amethod according to claim 4 wherein the milling is carried out for aperiod of from 2 to 8 hours.